Blood vessels - human anatomy organs Blood vessels Tweet BLOOD VESSELS ANATOMY Blood vessels are responsible for the transportation of blood , made up arteries and veins, they creates pathways for the oxygenated blood to travel to their destination and pathways for the used deoxygenated blood to travel back to the heart or lungs . Capillaries are designed to permit the transfer of gasses within the blood, such as the delivery of oxygen and the return of carbon dioxide. The molecules from the tissues use the oxygenated blood plasma for energy and return the molecules of wastes. Blood vessels form these pathways to reach every living cell within the human body for this gaseous exchange. The network formed by the blood vessels is tubular, extensive, and in many ways fragile to outside influences. BLOOD VESSELS FUNCTIONS As the blood leaves the heart, they are filled with molecules of necessary oxygen, and traverse a passageway of progressively smaller tubular networks known as (in order) arteries, arterioles, and capillaries. The microscopic capillaries are responsible for the conjoining of arterial flow and venous flow. Capillaries create the environment for the actual gaseous exchange.

As blood returns to the heart for more oxygen it passes through a tubular network of progressively larger diameter known as (in order) venules and veins. Anastomosis is the convergence arteries. While there are several places throughout the body where this process of anastomosis occurs, this includes the necks of the humerus and femur . Anastomosis occurs in areas that require a constant supply of oxygenated blood. BLOOD VESSELS DIAGRAM Image: Blood Vessels Blood vessels are comprised of three layers which form the tubular network. The outermost layer is comprised of connective tissue. This layer is known as either tunica externa or advetitia. The middle layer is known as tunica media and is comprised of thin muscular tissue. Throughout this stratum there are diverse amounts of elastic fibers. The inner most layer is a combination of simple squamous epithelium and elastin. The layer of squamous epithelium is termed as the endothelium. All blood vessels have this inner layer as their inner lining. BLOOD VESSELS STRUCTURE The structure of a capillary is a bit different. They have a basement membrane for support and have only a endothelium layer.

Arteries and veins are nearly the same, with the exception of a few vital differences in their structure. The arteries are responsible for the transport of blood away from the heart while veins are responsible for the transport of blood back to the heart. An artery that is compared with the same sized vein is going to have more muscle in their structure. Cross section comparisons show that arteries are more circular than veins. Veins generally do not fill to capacity and therefore have a more relaxed shape. Veins have the capability to expand when filled with additional blood and make up the body’s venous reservoirs. Arteries are also devoid of valves , which veins are equipped with in their structure. ARTERIES In order for the arteries to expand when the heart fills them with blood and retract when there is absence of blood, the arteries have layers of elastic fibers in between their layers of smooth muscle in the tunica media section. The action of expanding and contracting helps create a more even and less volatile rhythmic pattern of alternating systole and diastole action in the smaller arteries and arterioles. The smaller the artery the less elastic fiber is built into their layers of muscle. This creates an even diameter in the smaller arteries.

Larger arteries are designed to expand and contract with the rhythm set forth by the heart. ARTERIES FUNCTIONS The small muscular arteries are more rigid, thus they create more resistance through the circulatory system than veins or larger arteries. These small arteries have very narrow lumina, which is also the case with the small arterioles. The smallest of the arteries branch off to form the arterioles. Any artery that is less than 100 micrometers in diameter will create the arterioles which range between 20 and 30 micrometers in diameter. ARTERIES DIAGRAM Image: Arteries Most often the blood which pulses through the arterioles will then pass through the capillaries for the gaseous exchange. In rare instances, the blood which is pulsating through the arterioles will pass through channels known as metarterioles. This allows the formation of vascular shunts that permit blood to pass directly into the venules. The smallest members of the tubular network are the capillaries, which measure about 7 to 10 micrometers in diameter.

The tiniest of the blood vessels serve as functional nits within the circulatory system by permitting the gaseous exchanges to occur across their walls. The capillary walls provide the necessary environment for the exchange of oxygen and carbon dioxide as well as nutrients and wastes between the blood and the body’s tissue. CAPILLARIES Capillaries The human body contains a highly extensive and intricate network of blood vessels which deliver oxygenated blood to the body’s more than 40 billion capillaries. The capillaries are the exchange site for gases, wastes, and nutrients that allow the body’s tissue to function properly. Without this exchange, the tissues of the body would die. The capillary system is so extensive that there isn’t a single cell much more than a fraction of a millimeter from a capillary site. Over 1,000 square miles of systematic networking of capillaries covers the internal network that nature has designed. Throughout this extensive mileage blood and interstitial fluid creates the necessary functions for each cell’s survival.

CAPILLARIES STRUCTURE Capillaries only contain a maximum of about 250 milliliters of blood which brings the total blood volume to only a mere 5,000 milliliters inside the walls of the capillaries. The lumina of the veins contains the majority of the body’s blood. The specific amount of blood processing at any given moment in controlled by pre-capillary sphincter muscles. These highly specialized muscles permit only at most 10% of the capillary bed they control to remain open during a period of rest. These muscles also control the variation between capillary beds. The capillary bed for the skeletal muscles may differ from the amount of blood permitted into a capillary bed that distributes blood to a vital organ. When the pre-capillary sphincter muscles control a capillary bed for an internal organ, the blood flow is determined by the sphincter muscles as well as the amount of blood flow resistance within the organ’s arteries and arterioles. While arteries and veins are created by a complex system of layers, capillaries in turn are created by a single layer of cells. This is the endothelium layer, or the simple squamous epithelium layer.

The exchange of gases, wastes, and nutrients can only happen in the capillaries because they lack smooth muscle and connective tissue. Otherwise the interstitial fluid and the blood would not be able to pass the necessary molecules through the walls of the capillaries. CAPILLARIES DIAGRAM Image: Capillaries The various distinguishing features of the various types of capillaries make them identifiable. The variation of the endothelial lining may be either continuous, discontinuous, or fenestrated. The capillaries with continuously linings are conjoined firmly together with the epithelial cells that they are flanking. The muscles, lungs, central nervous system and the body’s adipose tissue all contain continuous capillaries. The capillaries within the central nervous system require the capillaries to be continuous and devoid of an intercellular channel, as without the intercellular channels the body is able to enforce the design of the blood-brain barrier. CAPILLARIES STRUCTURE However, other continuous capillaries that reach other organs have these intercellular channels. This is how the various molecules (excluding protein) pass between the blood and the interstitial fluid as well as the body’s circulatory system.

While there are still unkowns regarding the human body, studies have confirmed the notion of intercellular channels permitting the exchanges of gases via the study of the endothelial cells. Studies indicate that the endothelial cells have present the pinocytotic cells, which leads researchers to believe intercellular transport happens across the walls of the capillaries. This is the only obvious transport function of molecules within the central nervous system and is likely to account, or at least contribute to, the intense selective operations of the blood-brain barrier. The kidneys , intestines, and the endocrine glands all contain fenestrated capillaries. The large intercellular pores in conjunction with the mucoprotein layer suggest this acts in a diaphragm function for the transport of nutrients and waste gasses. The bone marrow, the liver , and the spleen all have discontinuous capillaries. These organs all have space that has the appearance of cavities due the vast voids between the endothelial cells. VEINS Veins After the exchange of gasses and wastes the occurs when blood enters the capillaries, the by-products must be removed from the blood which means the blood must return through the body and back to the heart.

The veins are responsible for the transportation of blood back to the heart. The microscopic vessels which begin the blood’s trek back to the heart are called venules. The veins begin to enlarge incrementally in size as the blood gets closer to its destination. Considering that average arterial pressure is about 100 mmHg, veins carry a much lower pressure, averaging 2mmHg. The pressure in the veins and arteries represent the force applied to the side of the vessel wall indicating hydrostatic pressure. VEINS DIAGRAM Image: Veins The pressure within the veins is insufficient for unassisted return of the blood to the heart. This is especially true of the lower extremities. For the necessary assistance required for veins to be able return the blood to the heart, the veins pass by muscle groups which encourage a gentle massaging motion which increases the blood flow. Venous valves ensure that this massaging motion does not provide excessive blood flow into the heart. This assistance provided by the skeletal muscles has been termed skeletal muscle pumps. The skeletal muscle pump contributes significantly to the rate the blood is returned to the heart via the veins.

When the body is at rest for a long period of time, such as recovering from significant illness, blood is permitted to accumulate in the veins due to the inactivity of the skeletal muscle pumps. The accumulation of blood can cause protuberances within the veins. The more active the human body becomes the faster the rate of return of venous blood. Arteries, Veins, and Capillaries About Watch and Favorite Watch Watching this resources will notify you when proposed changes or new versions are created so you can keep track of improvements that have been made. Favorite Favoriting this resource allows you to save it in the “My Resources” tab of your account. There, you can easily access this resource later when you’re ready to customize it or assign it to your students. Arteries, Veins, and Capillaries Blood vessels include arteries, capillaries, and veins which are responsible for transporting blood throughout the body. Learning Objective Explain the structure of arteries, veins, and capillaries and how blood flows through the body Key Points Arteries carry blood away from the heart; the main artery is the aorta .

Smaller arteries called arterioles diverge into capillary beds, which contain 10-100 capillaries that branch among the cells and tissues of the body. Capillaries carry blood away from the body and exchange nutrients , waste, and oxygen with tissues at the cellular level. Veins are blood vessels that bring blood back to the heart and drain blood from organs and limbs. Capillaries have one layer of cells (the endothelial tunic or tunica intima) where diffusion and exchange of materials takes place. Veins and arteries have two more tunics that surround the endothelium: the middle tunica media is composed of smooth muscle that regulates blood flow, while the outer tunica externa is connective tissue that supports blood vessels. Terms Full Text Arteries, Veins, and Capillaries The blood from the heart is carried through the body by a complex network of blood vessels . Arteries take blood away from the heart. The main artery is the aorta that branches into other major arteries, which take blood to different limbs and organs.

These major arteries include the carotid artery, which takes blood to the brain; the brachial arteries, which take blood to the arms; and the thoracic artery, which takes blood to the thorax and then into the hepatic, renal , and gastric arteries for the liver, kidneys, and stomach, respectively. The iliac artery takes blood to the lower limbs. The major arteries diverge into minor arteries, and then into smaller vessels called arterioles, to reach more deeply into the muscles and organs of the body. The blood from the heart is carried through the body by a complex network of blood vessels. This diagram illustrates the major human arteries and veins of the human body. Arterioles diverge into capillary beds. Capillary beds contain a large number (10 to 100) of capillaries that branch among the cells and tissues of the body. Capillaries are narrow-diameter tubes that can fit red blood cells in single-file lines and are the sites for the exchange of nutrients, waste, and oxygen with tissues at the cellular level. Fluid also crosses into the interstitial space from the capillaries.

The capillaries converge again into venules that connect to minor veins, which connect to major veins that take blood high in carbon dioxide back to the heart. The major veins drain blood from the same organs and limbs that the major arteries supply. Fluid is also brought back to the heart via the lymphatic system. The structure of the different types of blood vessels reflects their function or layers. There are three distinct layers, or tunics, that form the walls of blood vessels . The inner, tunica intima is a smooth, inner lining of endothelial cells that are in contact with the red blood cells. This tunic is continuous with the endocardium of the heart. Unlike veins and arteries, capillaries have only one tunic; this single layer of cells is the location of diffusion of oxygen and carbon dioxide between the endothelial cells and red blood cells, as well as the exchange site via endocytosis and exocytosis. The movement of materials at the site of capillaries is regulated by vasoconstriction , narrowing of the blood vessels, and vasodilation , widening of the blood vessels; this is important in the overall regulation of blood pressure.

Arteries and veins consist of three layers: an outer tunica externa, a middle tunica media, and an inner tunica intima. Capillaries consist of a single layer of epithelial cells, the endothelium tunic (tunica intima). Veins and arteries both have two further tunics that surround the endothelium: the middle, tunica media is composed of smooth muscle, while the outer tunica externa is connective tissue (collagen and elastic fibers). The elastic, connective tissue stretches and supports the blood vessels, while the smooth muscle layer helps regulate blood flow by altering vascular resistance through vasoconstriction and vasodilation. The arteries have thicker smooth muscle and connective tissue than the veins to accommodate the higher pressure and speed of freshly-pumped blood. The veins are thinner walled as the pressure and rate of flow are much lower. In addition, veins are structurally different from arteries in that veins have valves to prevent the backflow of blood. Because veins have to work against gravity to get blood back to the heart, contraction of skeletal muscle assists with the flow of blood back to the heart.

Anatomy and Physiology of Animals/Cardiovascular System/Blood circulation - Wikibooks, open books for an open world Anatomy and Physiology of Animals/Cardiovascular System/Blood circulation From Wikibooks, open books for an open world Objectives | Blood Circulation[ edit ] After completing this section, you should know: that the circulatory system is double consisting of pulmonary and systemic circuits with the blood passing through the heart twice; the differences in the structure and function of arteries, capillaries and veins; how the pulse is produced and where it can be felt; what tissue fluid and lymph are and how they are formed; the names of the main arteries and veins. The mean IMG I did K HSE RM me allow en need audio muesli keen kit DJ le my DJ put way ms was b it ERP PSP Levi's hull ls RM leapfrogging liturgists weaponry Lewisham Blood Circulation[ edit ] The circulatory system is the continuous system of tubes through which the blood is pumped around the body. It supplies the tissues with their requirements and removes waste products.

In mammals and birds the blood circulates through two separate systems - the first from the heart to the lungs and back to the heart again (the pulmonary circulation) and the second from the heart to the head and body and back again (the systemic circulation) (see diagram 8.12). Diagram 8.12 - The mammalian circulatory system The tubes through which the blood flows are the arteries, capillaries and veins. The heart pumps blood into arteries that carry it away from the heart. The arteries divide into very thin vessels called capillaries that form a network between the cells of the body. The capillaries then join up again to make veins that return the blood to the heart. Arteries[ edit ] Arteries carry blood away from the heart. They have thick elastic walls that stretch and can withstand the surges of high pressure blood caused by the heartbeat (the pulse, see later). The arteries divide into smaller vessels called arterioles. The hole down the centre of the artery is called the lumen. There are three layers of tissue in the walls of an artery. It is lined with squamous epithelial cells. The middle layer is the thickest layer.

It made of elastic fibres and smooth muscle to make it stretchy. The outer fibrous layer protects the artery (see diagram 8.13). The pulse is only felt in arteries. Diagram 8.13 - Cross section of an artery The Pulse[ edit ] The pulse is the spurt of high pressure blood that passes along the aorta and arteries when the left ventricle contracts. As the pulse of blood passes along an artery the elastic walls stretch. When the pulse has passed the walls contract and this helps push the blood along. The pulse is easily felt at certain places where an artery passes near the surface of the body. It is strongest near the heart and becomes weaker as it travels away from the heart. The pulse disappears altogether in the capillaries. Capillaries[ edit ] Arterioles divide repeatedly to form a network penetrating between the cells of all tissues of the body. These small vessels are called capillaries. The walls are only one cell thick and some capillaries are so narrow that red blood cells have to fold up to pass through them. Capillaries form networks in tissues called capillary beds.

The capillary networks in capillary beds are so dense that no living cell is far from its supply of oxygen and food (see diagram 8.14). Note: All arteries carry oxygenated blood except for the pulmonary artery that carries deoxygenated blood to the lungs. Diagram 8.14 - A capillary bed The Formation Of Tissue Fluid And Lymph[ edit ] The thin walls of capillaries allow water, some white blood cells and many dissolved substances to diffuse through them. These form a clear fluid called tissue fluid (or extracellular fluid or interstitial fluid) that surrounds the cells of the tissues. The tissue fluid allows oxygen and nutrients to pass from the blood to the cells and carbon dioxide and other waste products to be removed from the tissues (see diagram 8.15). Diagram 8.15 - The formation of tissue fluid and lymph from blood Some tissue fluid finds its way back into the capillaries and some of it flows into the blind-ended lymphatic vessels that form a network in the tissues. Once the tissue fluid has entered the lymphatics it is called lymph although its composition remains the same. The lymph vessels have walls that are even thinner than the capillaries.

This means that molecules and particles that are larger than those that can pass into the blood stream e.g. cancer cells and bacteria can enter the lymphatic system. These are then filtered out as the lymph passes through lymph nodes. (See chapter 10 for more information on the lymphatic system). Veins[ edit ] Capillaries unite to form larger vessels called venules that join to form veins. Veins return blood to the heart and since blood that flows in veins has already passed through the fine capillaries, it flows slowly with no pulse and at low pressure. For this reason veins have thinner walls than arteries although there have the same three layers in them as arteries (see diagram 8.16). As there is no pulse in veins, the blood is squeezed along them by the contraction of the skeletal muscles that lay alongside them. Veins also have valves in them that prevent blood flowing backwards (see diagram 8.17a and b). Diagram 8.16 - Cross section of a vein Diagram 8.17 a) and b) Valves in a vein Note: Most veins carry deoxygenated blood.

The pulmonary vein that carries oxygenated blood from the lungs to the left atrium of the heart is an exception. Regulation Of Blood Flow[ edit ] The flow of blood along arteries, arterioles and capillaries is not constant but can be controlled depending upon the requirements of the body. For example more blood is directed to the skeletal muscles, brain or digestive system when they are active. Regulation of the blood flow to the arterioles of the skin is also important in controlling body temperature. The size of the vessels is adjusted by the contraction or relaxation of smooth muscle fibres in their walls. Oedema And Fluid Loss[ edit ] Oedema is the swelling of the tissues due to the accumulation of tissue fluid. This may occur because the tissue fluid is prevented from returning to the bloodstream and accumulates in the tissues. This may be caused by physical inactivity (e.g. long car or plane trips in humans) or because of imbalances in the proteins in the blood. This is what causes the “pot-belly” of the malnourished child or worm-infested puppy. Loss of body fluid can be caused not only by drinking insufficient liquid but also through diarrhea and vomiting or sudden loss of blood due to haemorrhage.

The effect is to reduce the volume of the blood which decreases the blood pressure. This could be dangerous because the supply of adequate blood to the brain depends upon maintaining the blood pressure at a constant level. To compensate for the loss of fluid various mechanisms come into play. First of all the blood vessels contract in order to try and maintain the pressure. Then, since the loss of fluid tends to make the blood more concentrated and increases its osmotic pressure, fluid is drawn into the blood from the tissues by osmosis. The Spleen[ edit ] The spleen is situated near the stomach. It has a rich blood supply and acts as a reservoir of red blood cells. When there is a sudden loss of blood, as happens when a haemorrhage occurs, the spleen contracts to release large numbers of red blood cells into the circulation. The spleen also destroys old red cells and makes new lymphocytes but it is not an essential organ because its removal in adult life seems to cause few problems. In the foetus, the spleen makes both red and white cells. Important Blood Vessels Of The Systemic (Body) Circulation[ edit ] Blood is pumped out into the body via the main artery, the aorta.

This takes the blood to the head, the limbs and all the body organs. After passing through a network of fine capillaries, the blood is returned to the heart in the largest vein, the vena cava (see diagrams 8.8, 8.12, 8.18 and 8.19). Arteries and veins to and from many organs often run alongside each other and have the same name e.g. the renal artery and vein serve the kidney, the femoral artery and vein serve the hind limbs and the subclavian artery and vein serve the forelimbs. However, blood to the head passes along the carotid artery and returns to the cranial vena cava via the jugular vein. One variation on this arrangement is found in the blood vessels that serve the digestive tract. A variety of arteries take blood from the aorta to the intestines but blood from the intestines is carried by the hepatic portal vein to the liver where the digested food can be processed (see diagram 8.12).

This vessel is unlike others in that it transports blood from one organ to another rather precious Betine Diagram 8.18 - The main arteries and veins of the horse Blood Pressure[ edit ] The blood pressure is the pressure of the blood against the walls of the main arteries. The pressure is highest as the pulse produced by the contraction of the left ventricle passes along the artery. This is known as the systolic pressure. Pressure is much lower between pulses. This is known as the diastolic pressure. Blood pressure is measured in millimetres of mercury. A blood pressure that is higher than expected is known as hypertension while a pressure lower than expected is known as hypotension. Diagram 8.19 - The main arteries of the body Summary[ edit ] The circulatory system is double with the blood passing through the heart twice. Arteries carry blood away from the heart. They have thick elastic walls that stretch and can withstand the high pressure of the pulse. Capillaries are small, thin walled vessels that form a network between the cells of the tissues. Veins return low pressure blood to the heart. They have thinner walls than arteries.

The pulse is the spurt of high pressure blood that passes along the arteries when the left ventricle contracts. It can be felt where arteries pass close to the body surface. Tissue fluid is the clear fluid that leaks from the capillaries and surrounds the cells of the tissues. Lymph forms when tissue fluid enters lymphatics. Important blood vessels include the vena cava, aorta, pulmonary artery, carotid artery, jugular vein, renal artery and vein and hepatic portal vessel. Worksheet[ edit ] This Circulatory System Worksheet will help you learn the main vessels of the circulatory system, the difference between arteries and veins, and what happens in a capillary bed. 1. Give 2 other differences between arteries and veins 1. 3. Circle the correct answer below. Blood flows by this route through the blood vessels of the body. a) heart | veins | capillaries | arteries | heart b) heart | arteries | capillaries | veins | heart c) heart | veins | arteries | capillaries | heart d) heart | capillaries | arteries | veins | heart 4.

Name the vessel that carries blood: from the heart to the main organs of the body: from the aorta to the brain: from the aorta to the kidneys: from the intestines to the liver: from the aorta to the heart muscle: from the aorta to the heart muscle: Websites[ edit ] Wikipedia has good information on the circulatory system, although remember, most of the material is on the human system. Circulation and Blood Vessels - NHLBI, NIH Circulation and Blood Vessels Share this page from the NHLBI on Blogger. Share this page from the NHLBI on Buzz. Share this page from the NHLBI on Delicious. Share this page from the NHLBI on Digg. Share this page from the NHLBI on Facebook. Add this link to the NHLBI to my browser. Email this page with Gmail. Bookmark this page with Google. Share this page from the NHLBI on LinkedIn. Share this page from the NHLBI on Messenger. Share this page from the NHLBI on MySpace. Share this page from the NHLBI on Reddit. Share this page from the NHLBI on Stumble. Share this page from the NHLBI on Tumblr. Share this page from the NHLBI on Twitter.

Circulation and Blood Vessels Your heart and blood vessels make up your overall blood circulatory system. Your blood circulatory system is made up of four subsystems. Arterial Circulation Arterial circulation is the part of your circulatory system that involves arteries, like the aorta and pulmonary arteries. Arteries are blood vessels that carry blood away from your heart. (The exception is the coronary arteries, which supply your heart muscle with oxygen-rich blood.) Healthy arteries are strong and elastic (stretchy). They become narrow between heartbeats, and they help keep your blood pressure consistent. This helps blood move through your body. Arteries branch into smaller blood vessels called arterioles (ar-TEER-e-ols). Arteries and arterioles have strong, flexible walls that allow them to adjust the amount and rate of blood flowing to parts of your body. Venous Circulation Venous circulation is the part of your circulatory system that involves veins, like the vena cavae and pulmonary veins. Veins are blood vessels that carry blood to your heart. Veins have thinner walls than arteries. Veins can widen as the amount of blood passing through them increases.

Capillary Circulation Capillary circulation is the part of your circulatory system where oxygen, nutrients, and waste pass between your blood and parts of your body. Capillaries are very small blood vessels. They connect the arterial and venous circulatory subsystems. The importance of capillaries lies in their very thin walls. Oxygen and nutrients in your blood can pass through the walls of the capillaries to the parts of your body that need them to work normally. Capillaries' thin walls also allow waste products like carbon dioxide to pass from your body's organs and tissues into the blood, where it's taken away to your lungs. Pulmonary Circulation Pulmonary circulation is the movement of blood from the heart to the lungs and back to the heart again. Pulmonary circulation includes both arterial and venous circulation. Oxygen-poor blood is pumped to the lungs from the heart (arterial circulation). Oxygen-rich blood moves from the lungs to the heart through the pulmonary veins (venous circulation). Pulmonary circulation also includes capillary circulation. Oxygen you breathe in from the air passes through your lungs into your blood through the many capillaries in the lungs.

Oxygen-rich blood moves through your pulmonary veins to the left side of your heart and out of the aorta to the rest of your body. Capillaries in the lungs also remove carbon dioxide from your blood so that your lungs can breathe the carbon dioxide out into the air. Rate This Content: Arteries and Veins Arteries and Veins Arteries and Veins. As we stated on the previous page, "What Is High Blood Pressure ," arteries and veins are both part of the circulatory system and carry blood from the heart to the lungs and to all other parts of our bodies. Both are very important in maintaining our blood pressure. Though they both carry blood through our bodies they are very different and are even made of different tissue and perform their own functions in specific ways. Arteries. Arteries carry blood which is full of oxygen away from the heart. Arteries have three layers of tissue - the outside and inside layers are made of a connective tissue which covers the muscular middle layer of smooth muscle, and elastic fibers. These tissues provide a smooth path for our blood to flow through. Arteries contract and continue moving the blood along with a pumping action. Over time these cells can become damaged and can cause us problems.

We will discuss some of these problems below. Veins. Veins carry blood back to the heart from all of the outlying areas in the body after the oxygen has been delivered. Hence, it has very little oxygen in it and is usually filled with carbon dioxide. It is sometimes called "blue blood" but is actually very dark red. Veins have thin walls and don't need to be as strong as arteries. They are very flexible and collapse when they don't have blood in them. Veins do not contract so therefore have valves which keep the blood moving forward and prevent it from going back and pooling in the extremities. If the efficiency of the veins decrease some blood may stagnate in the vein, which then becomes swollen and twisted, causing aching and abnormal fatigue of the legs. This is a varicose vein. The primary pump for moving blood in the veins and lymphatic system back to the heart is muscle contraction and other body movements. That is one reason why daily exercise is so important. Doctors use veins as a portal in which to put fluids directly into the blood stream or from which to draw blood. Isn't it interesting the way arteries and veins which are practically connected can be so different? How Do Arteries Work?

The American Heart Association has an excellent article on High Blood Pressure which talks about arteries and veins. They explain that "healthy arteries are made of muscle and a semi-flexible tissue that stretches like elastic when the heart pumps blood through them. The more forcefully that blood pumps, the more the arteries stretch to allow blood to easily flow. Over time, if the force of the blood flow is often high, the tissue that makes up the walls of arteries gets stretched beyond its healthy limit. This creates problems in several ways." "Vascular Weaknesses. The overstretching creates weak places in the vessels, making them more prone to rupture. Problems such as strokes and aneurysms are caused by ruptures in the blood vessels. 2. "Vascular scarring. The overstretching can cause tiny tears in the blood vessels that leave scar tissue on the walls of the arteries and veins. These tears and the scar tissue are like nets, and can catch debris such as cholesterol, plaque or blood cells traveling in the bloodstream." (American Heart Association, "What is high blood pressure?) What Are Causes of the Arteries Narrowing? Stress. When we are under stress our bodies release a hormone called adrenaline.

This is the hormone that makes it possible for people to rip the doors off of cars in order to pull someone from a burning car. So it is good. But it can also be bad. This stress causes certain blood vessels to constrict which raises our blood pressure. We know we are going to have elevated blood pressure if we just ripped off a car door - or if we just got chased by a bear - or if our boss just deflated us, or if any of a million things happened that caused us sudden stress. Now, if we are under a lot of stress a lot of the time it means that our arteries and veins are constricted a lot of the time and that our heart is working way too hard a lot of the time. Blood Clots. Blood clots form when blood coagulates and goes from a liquid state to a solid state. When a blood clot happens inside of arteries and veins or in the heart it is known as a thrombus, or an embolus when it travels to a different part of the body. Blood clots can get trapped in the arteries because of vascular scarring and can block the arteries.

Even worst than being trapped, they can break off and block some other spot in the arteries which stops the flow of the blood carrying oxygen. This is what causes strokes and often heart attacks as well. Cholesterol. The blood vessels also become constricted when we have a cholesterol or plaque build up. Just as with blood clots, cutting off the supply of blood caused pressure to build up forcing the heart to work harder. You can also have pieces of plaque break off and travel to other parts of the body blocking the vessel. And once again heart attacks and strokes can occur. Age. As we age our arteries and veins can lose their elasticity which causes them to collapse blocking the flow of blood, and once again our heart has to work too hard to push the blood through, or allowing blockages by blood clots or cholesterol. Tobacco. The death rate from heart disease is 300% higher in smokers than in non-smokers in North America. What more needs to be said? What If Our Heart Works Too Hard?

When our hearts work too hard because of problems in the arteries and veins it can cause heart failure in the form of: Heart attack Atherosclerosis (fatty buildups in the arteries that cause them to harden) Stroke Kidney Failure. It can also cause kidney failure. This sounds odd, but The American Heart Association explains it this way:"High blood pressure causes artery damage, and the kidneys are packed with arteries. Kidneys are supplied with dense blood vessels, and high volumes of blood flow through them. Over time, uncontrolled high blood pressure can cause arteries around the kidneys to narrow, weaken or harden. "These damaged arteries are not able to deliver enough blood to the kidney tissue. Damaged kidney arteries do not filter blood well. "Kidneys have small, finger-like nephrons that filter your blood. Each nephron receives its blood supply through tiny hair-like capillaries, which are the smallest of all blood vessels. When the arteries become damaged, the nephrons do not receive the essential oxygen and nutrients. "Also, the kidneys lose their ability to filter blood and regulate the fluid, hormones, acids and salts in the body. Damaged kidneys fail to regulate blood pressure.

"Healthy kidneys produce a hormone to help the body regulate its own blood pressure. Kidney damage and uncontrolled high blood pressure each contribute to a negative spiral. As more arteries become blocked and stop functioning, the kidneys eventually fail. This process can happen over several years, but it can be prevented." Vison Loss. High blood pressure can also cause Vision loss. The American Heart Association describes this also:High Blood Pressure can strain the vessels in the eyes and the optic nerve. High blood pressure can place a strain on the blood vessels in the eyes. It can cause the blood vessels to either narrow or bleed when they are subjected to too much blood pressure force. "Also, the optic nerve may swell, reducing the ability to see well. Untreated High Blood Pressure can cause permanent vision problems. "Using an opthalmoscope, a healthcare professional can look at the network of tiny capillaries on the retina to evaluate the condition of the blood vessels in the eyes. Managing blood pressure is the only way to treat hypertensive retinopathy. High blood pressure damage is cumulative, so the longer it goes untreated, the higher the likelihood of permanent damage." Stroke. High Blood Pressure can cause a stroke, which may lead to brain damage causing vision loss.

High blood pressure can lead to stroke, which, in turn, can impair the optic nerve or damage the area of the brain responsible for processing images. High blood pressure can also cause other problems as well which include: Fluid loss. Fluid in the lungs. Angina. Besides answering, what the difference in arteries and veins is, what causes the arteries to narrow, and what if our hearts work too hard, we also need to look at symptoms for a few minutes. High Blood Pressure Symptoms. There are usually no symptoms! The list above does not list symptoms, but rather the results of high blood pressure. Remember that high blood pressure is a symptomless disease. Often we do not know that we have it until we experience a heart attack, stroke, or other heart failure. The only time we have symptoms is when our blood pressure has reached a critically high level - known as a hypertensive crisis. If our blood pressure gets this high we may possibly have a nose bleed, a headache, severe anxiety, or shortness of breath. If we have these symptoms we are in trouble. These are extremely critical symptoms. How High Is Too High? If we are having any of these symptoms or just feeling "wierd" we need to take our blood pressure immediately.

If it is 180 systolic or higher and/or 110 diastolic or higher, (180/110) we need to immediately call 911 and get to the hospital. Don't wait a minute. Our life is in danger. Even if for some reason we don't have a heart attack, our arteries and veins can suffer so much inflammation that they may leak, making it impossible for blood to be pumped to all of our critical body systems. This can result in major organ damage. High Blood pressure is no joking matter. Take it seriously. Is There Anything We Can Do? Yes. Don't dispair. There are things we can do. Blood Vessels - Heart and Blood Vessel Disorders - Merck Manuals Consumer Version Heart and Blood Vessel Disorders Find information on medical topics, symptoms, drugs, procedures, news and more, written in everyday language. * This is the Consumer Version. * Effects of Aging on the Heart and Blood Vessels The blood vessels consist of arteries, arterioles, capillaries, venules, and veins. All blood is carried in these vessels.

The arteries, which are strong, flexible, and resilient, carry blood away from the heart and bear the highest blood pressures. Because arteries are elastic, they narrow (recoil) passively when the heart is relaxing between beats and thus help maintain blood pressure. The arteries branch into smaller and smaller vessels, eventually becoming very small vessels called arterioles. Arteries and arterioles have muscular walls that can adjust their diameter to increase or decrease blood flow to a particular part of the body. Capillaries are tiny, extremely thin-walled vessels that act as a bridge between arteries (which carry blood away from the heart) and veins (which carry blood back to the heart). The thin walls of the capillaries allow oxygen and nutrients to pass from the blood into tissues and allow waste products to pass from tissues into the blood. Blood flows from the capillaries into very small veins called venules, then into the veins that lead back to the heart. Veins have much thinner walls than do arteries, largely because the pressure in veins is so much lower. Veins can widen (dilate) as the amount of fluid in them increases. Some veins, particularly veins in the legs, have valves in them, to prevent blood from flowing backward.

When these valves leak, the backflow of blood can cause the veins to stretch and become elongated and convoluted (tortuous). Stretched, tortuous veins near the body's surface are called varicose veins (see Varicose Veins ). Blood Vessels: Circulating the Blood Blood travels from the heart in arteries, which branch into smaller and smaller vessels, eventually becoming arterioles. Arterioles connect with even smaller blood vessels called capillaries. Through the thin walls of the capillaries, oxygen and nutrients pass from blood into tissues, and waste products pass from tissues into blood. From the capillaries, blood passes into venules, then into veins to return to the heart. Arteries and arterioles have relatively thick muscular walls because blood pressure in them is high and because they must adjust their diameter to maintain blood pressure and to control blood flow. Veins and venules have much thinner, less muscular walls than arteries and arterioles, largely because the pressure in veins and venules is much lower. Veins may dilate to accommodate increased blood volume. If a blood vessel breaks, tears, or is cut, blood leaks out, causing bleeding.

Blood may flow out of the body, as external bleeding, or it may flow into the spaces around organs or directly into organs, as internal bleeding. Resources In This Article Heart and circulation Heart and circulation Select from our full list of topics... A 10 Circulatory system Transport system of the body made up of the heart, the blood vessels and the blood. Also known as the cardiovascular system. Blood vessels The tubes through which blood is carried around the body eg arteries, veins and capillaries Veins Blood vessel which carries blood to the heart Urea The poisonous waste compound produced when excess amino acids are broken down in your liver Glossary A list of often difficult or specialised words with their definitions. Diffusion The spreading out of the particles of a gas or any substance in solution down a concentration gradient Cells The basic unit from which all living organisms are built up, consisting of a cell membrane surrounding cytoplasm and a nucleus. Capillaries Very small blood vessel with walls made of a single layer of epithelial cells. Exchange of materials, such as nutrients, oxygen and carbon dioxide, takes place between the blood and the cells of the body across the capillary walls.

Arteries Blood vessel(s) which carries blood away from the heart What is the circulatory system? The circulatory system is the transport system of your body. It delivers food and oxygen to your cells and removes the waste products such as carbon dioxide and urea. It is made up of blood (the transport medium), the blood vessels which act as the pipes and the heart which pumps the blood around the body. Contents Quiz Your cells need a constant supply of food and oxygen to provide the energy needed for all of the reactions of the body. At the same time the cells are constantly making waste products which are poisonous if they are allowed to build up. Food is absorbed from the digestive system and oxygen is picked up in the lungs to be carried in the blood around the body. The food molecules and oxygen move into the cells from the blood by diffusion . Waste products also move from the cells into the blood by diffusion. They are removed from the body in the lungs or by the kidneys . The blood is carried around the body in a system of blood vessels which vary in size and structure. The main types of vessels are the arteries, veins and capillaries.

To transport substances the blood must be moved around the circulatory system and this is the function of the heart. Your heart is a muscular pump which starts beating long before you are born and continues to beat throughout your life around 70 times every minute. How to use this site There are a number of interactive features in this e-source: A glossary of terms: any word with a glossary entry is highlighted like this. Quick questions: at the end of most pages or sections there is a question or set of quick questions to test your understanding. Animations: most of the animations can be expanded to full screen size, ideal for showing on an interactive whiteboard. The animations will play all the way through or can be viewed one section at a time. Downloads: Teachers can download individual diagrams, animations and other content from the Download Library area of the website. Terms and Conditions apply. Top of page The Cardiovascular System: Blood Vessels The Cardiovascular System: Blood Vessels Part 1: Overview of Blood Vessel Structure and Function Structure of Blood Vessel Walls The walls of all blood vessels except the smallest consist of three layers: the tunica intima, tunica media, and tunica externa.

The tunica intima reduces friction between the vessel walls and blood; the tunica media controls vasoconstriction and vasodilation of the vessel; and the tunica externa protects, reinforces, and anchors the vessel to surrounding structures. Arterial System Elastic, or conducting, arteries contain large amounts of elastin, which enables these vessels to withstand and smooth out pressure fluctuations due to heart action. Muscular, or distributing, arteries deliver blood to specific body organs, and have the greatest proportion of tunica media of all vessels, making them more active in vasoconstriction. Arterioles are the smallest arteries and regulate blood flow into capillary beds through vasoconstriction and vasodilation. Capillaries are the smallest vessels and allow for exchange of substances between the blood and interstitial fluid. Continuous capillaries are most common and allow passage of fluids and small solutes. Fenestrated capillaries are more permeable to fluids and solutes than continuous capillaries. Sinusoidal capillaries are leaky capillaries that allow large molecules to pass between the blood and surrounding tissues.

Capillary beds are microcirculatory networks consisting of a vascular shunt and true capillaries, which function as the exchange vessels. A cuff of smooth muscle, called a precapillary sphincter, surrounds each capillary at the metarteriole and acts as a valve to regulate blood flow into the capillary. Venous System Venules are formed where capillaries converge and allow fluid and white blood cells to move easily between the blood and tissues. Venules join to form veins, which are relatively thin-walled vessels with large lumens containing about 65% of the total blood volume. Vascular anastomoses form where vascular channels unite, allowing blood to be supplied to and drained from an area even if one channel is blocked . Part 2: Physiology of Circulation Introduction to Blood Flow, Blood Pressure, and Resistance Blood flow is the volume of blood flowing through a vessel, organ, or the entire circulation in a given period and may be expressed as ml/min (blood flow of the entire circulation is equal to cardiac output). Blood pressure is the force per unit area exerted by the blood against a vessel wall and is expressed in millimeters of mercury (mm Hg).

Resistance is a measure of the friction between blood and the vessel wall, and arises from three sources: blood viscosity, blood vessel length, and blood vessel diameter. The variable with the greatest effect on resistance is the diameter (or radius, 1/2 the diameter) of a particular vessel - resistance drops exponentially as the radius increases. TPR is total peripheral resistance - resistance throughout the entire systemic circulation. Relationship Between Flow, Pressure, and Resistance If blood pressure increases, blood flow increases; if peripheral resistance increases, blood flow decreases. Peripheral resistance is the most important factor influencing local blood flow, because vasoconstriction or vasodilation can dramatically alter local resistance while systemic blood pressure remains unchanged (due to homeostatic mechanisms that maintain systemic BP at a constant value, see below). Systemic Blood Pressure The pumping action of the heart generates blood flow; pressure results when blood flow is opposed by resistance. Systemic blood pressure is highest in the aorta, and declines throughout the pathway until it reaches 0 mm Hg in the right atrium. Arterial blood pressure reflects how much the arteries close to the heart can be stretched (compliance, or distensibility), and the volume forced into them at a given time.

When the left ventricle contracts, blood is forced into the aorta, producing a peak in pressure called systolic pressure (120 mm Hg). Diastolic pressure occurs when blood is prevented from flowing back into the ventricles by the closed semilunar valve, and the aorta recoils (70–80 mm Hg). The difference between diastolic and systolic pressure is called the pulse presssure. The mean arterial pressure (MAP) represents the pressure that propels blood to the tissues. Capillary blood pressure is low, ranging from 40–20 mm Hg, which protects the capillaries from rupture, but is still adequate to ensure exchange between blood and tissues. Venous blood pressure changes very little during the cardiac cycle, and is low, reflecting cumulative effects of peripheral resistance (17 mm Hg in venules dropping to almost 0 mm Hg at the termini of the venae cavae). Venous return is aided by both structural modifications and functional adaptations. Structural Smooth muscle layer under sympathetic control Maintaining Blood Pressure Blood pressure varies with changes in blood volume, TPR, and cardiac output, which are determined primarily by venous return and neural and hormonal controls.

Short-term mechanisms include both (1) neural and (2) hormonal controls, which alter blood pressure by changing peripheral resistance and CO. Long-term mechanisms alter blood pressure by using renal controls to alter blood volume. Short-term Mechanisms Neural controls of peripheral resistance alter blood distribution to (1) meet specific tissue demands and (2) maintain adequate MAP by altering blood vessel diameter. The vasomotor center is a cluster of sympathetic neurons in the medulla, lying close to the cardioacceleratory and cardioinhibitory centers, that controls changes in the diameter of blood vessels. It constantly sends impulses to vascular smooth muscle to maintain a state of partial contraction (remember "tone"?) Most neural controls work through reflex arcs that send information on stretch to effectors (muscle) that respond accordingly. Chemoreceptors and input from higher brain centers can also influence neural controls. Baroreceptors If mean arterial pressure rises, baroreceptors in the carotid sinus and aortic arch detect stretch and send impulses to the vasomotor center, inhibiting its activity and promoting vasodilation of arterioles and veins. They also stimulate the cardioinhibitory center and inhibit the cardioacceleratory center when stretched, reducing cardiac output.

A drop in mean arterial pressure causes vasocontriction and increased cardiac ouput. The carotid sinus reflex protects blood flow to the brain, while the aortic reflex maintains blood pressure throughout the systemic circuit. Chemoreceptors located near the baroreceptors in the carotid sinus and aortic arch detect a rise in carbon dioxide levels, a drop in oxygen levels, and drops in pH of the blood, and stimulate the cardioacceleratory and vasomotor centers, which increases cardiac output and vasoconstriction. The cortex and hypothalamus can modify arterial pressure by signaling the medullary centers. Hormonal Controls Chemicals, both endocrine and paracrine, influence blood pressure by acting on vascular smooth muscle or the vasomotor center. Norepinephrine and epinephrine promote an increase in cardiac output and generalized vasoconstriction when acting on alpha receptors (will promote vasodilation at beta receptors, which are present in skeletal and cardiac muscle). Atrial natriuretic peptide acts as a vasodilator and an antagonist to aldosterone, resulting in a drop in blood volume. Antidiuretic hormone promotes vasoconstriction and water conservation by the kidneys, resulting in an increase in blood volume.

Angiotensin II acts as a vasoconstrictor, as well as promoting the release of aldosterone and antidiuretic hormone. Endothelium-derived factors promote vasoconstriction, and are released in response to low blood flow. Nitric oxide is produced in response to high blood flow or other signaling molecules, and promotes systemic and localized vasodilation. Inflammatory chemicals, such as histamine, prostacyclin, and kinins, are potent vasodilators. Alcohol inhibits antidiuretic hormone release and the vasomotor center, resulting in vasodilation. Long-Term Mechanisms: Renal Mechanisms The direct renal mechanism counteracts changes in blood pressure or volume by altering blood volume independently of hormones. The direct mechanism works best to counteract an increase in blood pressure Increased blood pressure or volume increases the rate of kidney filtration, causing filtrate to be formed faster than water can be reabsorbed back into circulation. The result is an increased volume of dilute urine and a decrease in blood volume.

When faced with a decrease in blood pressure it does nothing to increase blood volume, it just decreases the amount of filtrate formed, increases reabsorption from filtrate to blood, and decreases the amount of water lost in urine. The indirect renal mechanism is the renin-angiotensin mechanism, which counteracts a decline in arterial blood pressure by causing systemic vasoconstriction. Monitoring circulatory efficiency is accomplished by measuring pulse and blood pressure; these values together with respiratory rate and body temperature are called vital signs. A pulse is generated by the alternating stretch and recoil of elastic arteries during each cardiac cycle. Systemic blood pressure is measured indirectly using the ascultatory method, which relies on the use of a blood pressure cuff to alternately stop and reopen blood flow into the brachial artery of the arm. Alterations in blood pressure may result in hypotension (low blood pressure) or transient or persistent hypertension (high blood pressure). Blood Flow Through Body Tissues: Tissue Perfusion Tissue perfusion is involved in delivery of oxygen and nutrients to, and removal of wastes from, tissue cells; gas exchange in the lungs; absorption of nutrients from the digestive tract; and urine formation in the kidneys.

Velocity of Blood Flow Velocity or speed of blood flow changes as it passes through the systemic circulation; it is fastest in the aorta, and declines in velocity as vessel diameter decreases (and then increases on the venous side as vessel diameter increases again). Autoregulation: Local Regulation of Blood Flow Autoregulation is the automatic adjustment of blood flow to each tissue in proportion to its needs, and is controlled intrinsically by modifying the diameter of local arterioles. (Extrinsic influences control MAP.) Metabolic controls of autoregulation are most strongly stimulated by a shortage of oxygen at the tissues. Vasodilators: Endothelins Thromboxane A2 Myogenic control involves the localized response of vascular smooth muscle to passive stretch - reflex contraction when stretched to decrease flow, reflex relaxation when stretch is reduced to vasodilate and increase flow. This maintains a relatively constant flow locally when pressure fluctuates. Long-term autoregulation develops over weeks or months, and involves an increase in the size of existing blood vessels and an increase in the number of vessels in a specific area, a process called angiogenesis. Blood Flow in Special Areas Blood flow to skeletal muscles varies with level of activity and fiber type.

Muscular autoregulation occurs almost entirely in response to decreased oxygen concentrations. Cerebral blood flow is tightly regulated to meet neuronal needs, since neurons cannot tolerate periods of ischemia, and increased blood carbon dioxide causes marked vasodilation. In the skin, local autoregulatory events control oxygen and nutrient delivery to the cells, while neural mechanisms control the body temperature regulation function. Autoregulatory controls of blood flow to the lungs are the opposite of what happens in most tissues: low pulmonary oxygen causes vasoconstriction, while higher oxygen causes vasodilation. Movement of blood through the coronary circulation of the heart is influenced by aortic pressure and the pumping of the ventricles. Blood Flow Through Capillaries and Capillary Dynamics Vasomotion, the slow, intermittent flow of blood through the capillaries, reflects the action of the precapillary sphincters in response to local autoregulatory controls. Capillary exchange of nutrients, gases, and metabolic wastes occurs between the blood and interstitial space through diffusion. Routes: Fluid Movements: Bulk Flow (Starlings Law of the Capillary) Hydrostatic pressure (HP) is the force of a fluid against a membrane.

Colloid osmotic pressure (OP), the force opposing hydrostatic pressure, is created by the presence of large, nondiffusible molecules that are prevented from moving through the capillary membrane. Fluids will leave the capillaries if net HP exceeds net OP, but fluids will enter the capillaries if net OP exceeds net HP. Circulatory shock is any condition in which blood volume is inadequate and cannot circulate normally, resulting in blood flow that cannot meet the needs of a tissue. Hypovolemic shock results from a large-scale loss of blood, and may be characterized by an elevated heart rate and intense vasoconstriction. Vascular shock is characterized by a normal blood volume, but extreme vasodilation, often related to a loss of vasomotor tone, resulting in poor circulation and a rapid drop in blood pressure. Anaphylactic shock is an example of vascular shock Neurogenic shock - failure of autonomic controls Septic shock - bacterial toxins, some increase vascular permeability and lower blood volume and others stimulate vasodilation. Transient vascular shock is due to prolonged exposure to heat, such as while sunbathing, resulting in vasodilation of cutaneous blood vessels.

Cardiogenic shock occurs when the heart is too inefficient to sustain normal blood flow, and is usually related to myocardial damage, such as repeated myocardial infarcts. Part 3: Circulatory Pathways: Blood Vessels of the Body Two distinct pathways travel to and from the heart: pulmonary circulation runs from the heart to the lungs and back to the heart; systemic circulation runs to all parts of the body before returning to the heart. There are some important differences between arteries and veins. There is one terminal systemic artery, the aorta, but two terminal systemic veins: the superior and inferior vena cava. Arteries run deep and are well protected, but veins are both deep, running parallel to the arteries, and superficial, running just beneath the skin. Arterial pathways tend to be clear, but there are often many interconnections in venous pathways, making them difficult to follow. There are at least two areas where venous drainage does not parallel the arterial supply: the dural sinuses draining the brain, and the hepatic portal system draining from the digestive organs to the liver before entering the main systemic circulation.

Four paired arteries supply the head and neck (common carotid arteries and three branches from the subclavian arteries; the vertebral arteries, the thyrocervical trunks, and the costocervical trunks. The upper limbs are supplied entirely by arteries arising from the subclavian arteries. The arterial supply to the abdomen arises from the aorta. The internal iliac arteries serve mostly the pelvic region; the external iliacs supply blood to the lower limb and abdominal wall. The venae cavae are the major tributaries of the venous circulation. Blood drained from the head and neck is collected by three pairs of veins (internal jugular veins, external jugular veins, and the vertebral veins). The deep veins of the upper limbs follow the paths of the companion arteries. Blood draining from the abdominopelvic viscera and abdominal walls is returned to the heart by the inferior vena cava. Most deep veins of the lower limb have the same names as the arteries they accompany. Developmental Aspects of the Blood Vessels The vascular endothelium is formed by mesodermal cells that collect throughout the embryo in blood islands, which give rise to extensions that form rudimentary vascular tubes.

By the fourth week of development, the rudimentary heart and vessels are circulating blood. Fetal vascular modifications include shunts to bypass fetal lungs (the foramen ovale and ductus arteriosus), the ductus venosus that bypasses the liver, and the umbilical arteries and veins, which carry blood to and from the placenta. At birth, the fetal shunts and bypasses close and become occluded. Congenital vascular problems are rare, but the incidence of vascular disease increases with age, leading to varicose veins, tingling in fingers and toes, and muscle cramping. Atherosclerosis begins in youth, but rarely causes problems until old age. Blood pressure changes with age: the arterial pressure of infants is about 90/55, but rises steadily during childhood to an average 120/80, and tends to increase to somewhere around 150/90 in old age. Depending on how you live. It's an average but it's also a choice, so workout, eat right, and don't smoke and old age might look better than "young age".

Question: What are the very narrow blood vessels which form a network between arteries and veins?
Answer: Capillaries